1. Field of the Invention
The present invention relates generally to a method and equipment for cleaning a substrate and, more particularly, to a substrate cleaning method and equipment for removing infinitesimal foreign matters (dusts, particles etc.) adhered to a glass substrate such as a photomask and a reticle for manufacturing a semiconductor.
2. Related Background Art
A line width of a pattern depicted on the surface of a semiconductor wafer substrate has become increasingly hyperfine. In the case of obtaining the hyperfine pattern by photolithography, if foreign matters such as dusts are adhered onto the photomask or the reticle, the foreign matters act as light shielding or scattering substances. In general, the circuit pattern is formed by opaque material like chromium, or phase shift material on the surface of a transparent substrate.
For this reason, the image of the foreign matters is transferred onto the wafer, and it follows that pattern different from the original mask pattern is formed on the wafer.
As a result, there exists a possibility in which an external configuration of the mask pattern is deteriorated, and further a defect on the circuit is caused. An undesirable influence by the deterioration of the external configuration increases when forming a higher integration of the circuit or a more hyperfine pattern. For this reason, the deterioration of the external configuration due to the infinitesimal foreign matters is not ignorable. Hence, an establishment of a method of substantially completely eliminating such foreign matters is of a high importance in terms of improving quality and yield of the product.
A conventional method and equipment for cleaning the photomask or the reticle are disclosed in U.S. Pat. No. 4,569,695. A brief explanation thereof will be given. The mask passes between a pair of rotary brushes while the surface and rear face of the mask are wetted with water or an electrolyte. Contaminations on the surface and rear face of the mask are thereby mechanically eliminated by the rotary brushes. The surface and rear face are rinsed with an organic solvent of an alcoholic system and thereafter dried in the steam of the alcoholic or freon system. A cleaning process is thus completed. This method is remarkably superior as a method of removing a foreign matter of 1 .mu.m or larger.
Further, a method of cleaning the substrate surface after making it hydrophilic is known in Japanese Patent Laid-Open Application No. 1-226156. This cleaning method involves the steps of irradiating the substrate surface with ultraviolet light in an oxidative atmosphere containing oxygen to make the substrate surface hydrophilic. Thereafter, a water-containing layer is formed on the substrate surface. A cleaning liquid is brought into close contact with the substrate surface formed with the water-containing layer, thus cleaning the substrate surface. Thereafter, the cleaned substrate surface is dried.
As a result of extensive investigation by the present inventors, it has been determined that infinitesimal dusts produced from the rotary brushes in the first-mentioned technique are re-adhered onto the glass substrate, so that foreign matters still remain on the glass substrate after cleaning. Hence, a sufficient effect for removing foreign matters as infinitesimal as 1 .mu.m or less was not necessarily obtained. It was also determined that in the second-mentioned technique, water stains appear after drying due to water that cannot be sufficiently removed in the drying step.